FIG. 1 shows the internal structure of a conventional hard disk device in a plan view, wherein the left side of a broken line shows the hard disk device in a state wherein the upper cover is removed, while the right side of the broken line shows the construction of a magnetic disk 11 and an arm assembly 12 that cooperates with the disk 11, wherein the magnetic disk 11 and the arm assembly 12 forms a part of a magnetic disk assembly 10 in which a plurality of magnetic disks are stacked with each other.
Referring to FIG. 1, each magnetic disk 11 is mounted upon a hub 11a that is driven by a motor not illustrated, and the arm assembly 12 includes a swing arm 12b held on a swing axle 12a and a magnetic head 12c provided on a free end of the arm 12b. Further, a coil 12d that forms a part of a voice coil motor 13 is provided on the arm 12b in correspondence to another free end, opposite to the first free end on which the magnetic head 12c is provided, wherein the coil 12d is wound parallel to the scanning surface of the arm 12b. Further, magnets 13a and 13b forming another part of the voice coil motor 13 are disposed above and below the coil 12d. Thereby, the arm 12 is rotated about the swing axle 12a freely in response to the energization of the coil 12d. The voice coil motor 13 is subjected to a servo control such that the magnetic head 12c carried on the arm 12b properly tracks a cylinder or track 1b that is defined on the magnetic disk 11.
FIG. 2 is a perspective view showing the internal structure of the hard disk device of FIG. 1.
Referring to FIG. 2, the magnetic disk assembly 10 includes a plurality of magnetic disks 11.sub.1, 11.sub.2, . . . that are held commonly on the rotary hub 11a, and the arm assembly 12 includes a plurality of arms corresponding to the plurality of magnetic disks. Each arm 12b is held on a common rotatable member 12e that in turn is held rotatable about the swing axle 12a, and all the arms 12b are swung simultaneously in response to the rotational motion of the member 12e. Of course, the member 12e is activated in response to the energization of the voice coil motor 13. Further, the entire structure of the hard disk device is accommodated within a hermetically sealed envelope 1.
In the hard disk devices having such a construction, one of the stacked magnetic disks such as the magnetic disk 11.sub.1 is recorded with a servo signal in the form of a magnetization pattern along a cylinder 11b (FIG. 1) that is defined on the magnetic disk. By controlling the magnetic head that cooperates with the magnetic disk 11.sub.1 to track the foregoing servo signal, other magnetic heads also track the cylinders on the respective, corresponding magnetic disks.
FIG. 3 shows an example of the servo control signal recorded on a magnetic disk 11.sub.1.
Referring to FIG. 3, the servo control signal is recorded in the form of a plurality of concentric magnetic stripes M.sub.1, M.sub.2, . . . , wherein FIG. 3 shows only a part of the five, consecutive magnetic stripes. It will be noted that the cylinder 11b is formed in correspondence to the boundary between the magnetic stripes M.sub.1, M.sub.2, . . . as designated as CY.sub.0, CY.sub.1, CY.sub.2, wherein each stripe M.sub.1, M.sub.2, . . . includes servo patterns P.sub.0, P.sub.1, P.sub.2 and P.sub.3 that are formed between two reference patterns REF. It will be noted that reference patterns REF are common in all of the magnetic stripes, while the servo patterns P.sub.0 -P.sub.3 are different in each magnetic stripe. In the illustrated example, the foregoing plurality of reference patterns include an N/S pattern that is repeated three times.
FIG. 4 shows a waveform of the servo signal reproduced by the magnetic head. As will be understood from the drawing, the reproduced servo signal changes depending upon the relative position of the magnetic head with respect to the cylinder. For example, it will be noted that the reproduced signal corresponding to the pattern P.sub.0 has a small reproduced level when the magnetic head is located on the cylinder CY.sub.0, while the reproduced signal corresponding to the same pattern P.sub.0 takes a maximum level when the magnetic head is located on the cylinder CY.sub.1.
FIG. 5 shows the level of two control signals POSN and POSQ extracted from the servo signals reproduced by the foregoing magnetic head, as a function of the relative position of the magnetic head with respect to the cylinders, wherein the signals POSN and POSQ have respective phases that are offset by 90 degrees with each other. The drawing indicates that one can determine the relative position of the magnetic head with respect to the three cylinders CY.sub.0, CY.sub.1 and CY.sub.2 from the peak position of the signal POSN or POSQ.
FIG. 6 is a block diagram of a control circuit used in the conventional hard disk device for positioning the magnetic head in accordance with the principle of FIG. 5.
Referring to FIG. 6, the servo signal reproduced by a magnetic head 12c is subjected to a gain control process in an AGC amplifier 21 such that the servo signals reproduced from the cylinders at an outer peripheral part of the magnetic disk have substantially the same gain as the servo signals reproduced from the cylinders at an inner peripheral part of the magnetic disk. The servo signals thus processed are supplied to a demodulator circuit 22, wherein the demodulator circuit 22 produces the foregoing two control signals POSN and POSQ having the respective phases that are offset from each other by 90 degrees, based upon the servo signals thus reproduced. Further, the demodulator circuit 22 produces a feedback signal based upon the level of the control signals POSN and POSQ and supplies the same to an AGC amplifier 21. The circuit 22 includes a resistor 22a for adjusting the amount of feedback of the foregoing feedback signal, and the AGC amplifier 21 achieves the foregoing gain adjustment based upon the feedback signal.
The control signals POSN and POSQ thus demodulated at the demodulator circuit 22 are then supplied to a selection circuit 23, wherein the selection circuit 23 extracts the linear part of the POSN and POSQ signals indicated in FIG. 5. Thereby, when the magnetic head 12c is located on the even number cylinders, the selection circuit 23 extracts the linear part of the POSN signal and outputs the same as a fine control signal FINS, while when the head 12c is located on the odd number cylinders, the switch circuit 23 extracts the linear part of the POSQ signal and outputs the same as the fine control signal FINS.
The control signals POSN and POSQ demodulated at the demodulation circuit 22 is then subjected to a differentiation at a speed detection circuit 24, wherein a speed signal V indicative of the real speed of the magnetic head is obtained based upon the slope of the foregoing linear part of the POSN or POSQ signal. Further, the control signals POSN and POSQ are converted to digital signals at an A/D converter 25 and supplied to a microprocessor 26 for coarse positional detection as well as for coarse speed detection. Thereby, the microprocessor 26 counts up the number of peaks of the POSN and POSQ signals with the scanning of the magnetic head and obtains the coarse position of the magnetic head 12c on the magnetic disk 11. Further, the microprocessor 26 produces a digital speed reference signal based upon the coarse position thus obtained and supplies the same to a comparator 27 after conversion to an analog speed reference signal V.sub.ref at a D/A converter 26. The comparator 27 is further supplied with the speed signal V detected at the speed detection circuit 24 and produces a control signal indicative of a difference between the signal V and the signal V.sub.ref. The control signal thus produced is then supplied to the voice coil motor 13 via a switch circuit 28 and a power amplifier 29.
In the foregoing control system, the microprocessor 26 controls the switch circuit 28, when it is detected that the magnetic head has reached in the vicinity of a desired cylinder as a result of the foregoing coarse positional detection, such that the fine control signal FINS from the selection circuit 23 is supplied to the voice coil motor 13 via the power amplifier 29. As a result, a precise positional control of the magnetic head is achieved based upon the peak position of the control signals POSN and POSQ as indicated in FIG. 5.
In such a conventional magnetic head control process, however, there arises a problem in that the variation in the width of the cores used in the magnetic head appears conspicuous particularly when the recording density on the magnetic disk has been increased. More specifically, the shape of the positional control signals POSN and POSQ shown in FIG. 7(A) tends to be modified as indicated in FIG. 7(C) depending upon the apparatus used. Thereby, it will be noted that the curves a and b, originally having an analogous shape except for the amplitude, which differs depending upon the core width, have mutually different slopes because of the fact that the AGC circuit 21 has fixed the peak level of the signals uniformly at a common level. It should be noted that the slope of the control signals POSN and POSQ is related to the sensitivity of the positional detection such that the sensitivity is relatively high in the case of the curve a while the sensitivity is low in the case of the curve b. The reason of the latter is that the signals POSN and POSQ have a flattened shape in the vicinity of the peaks. When the sensitivity of the positional detection is too high, the gain of the servo loop in the control circuit of FIG. 6 increases and there occurs a case wherein the gain exceeds 1 at a mechanical resonant point f.sub.0 as indicated in FIG. 7(D) when the magnetic head 12c has achieved a seek operation shown in FIG. 7(B). It should be noted that FIG. 7(B) shows the change of the POSN signal that occurs in response to the movement of the magnetic head 12c from the inner periphery of the magnetic disk 11 to the outer periphery of the magnetic disk 12. In FIG. 7(B), it will be noted that the peak-peak value of the signal is held at 4 volts as a result of the gain control achieved by the AGC amplifier 21 of FIG. 6. When the gain of the servo loop exceeds the level 1, the servo system causes an oscillation.
On the other hand, when the sensitivity of the control signals POSN and POSQ is low as indicated in FIG. 7(C) by a curve b, the gain of the servo loop decreases and the deviation of the magnetic head during the positional control increases. Further, the variation in the shape of the control signal as indicated in FIG. 7(C) tends to cause a deviation in the magnetic head speed V that is obtained by the circuit 24 with respect to the actual speed. Thereby, there is a substantial risk that one obtains an erroneous head position in the coarse positional control process achieved in response to the output of the comparator 27. FIG. 7(E) shows such a deviation in the detected magnetic head speed caused by the distortion of the shape of the control signals POSN and POSQ, wherein it will be noted that the linear part of the curve represented by a broken line has a slope that is different from the slope of the curve represented by a continuous line.
Thus, when there occurs an excessive error in the coarse positional control, there occurs problems such that the fine control of the magnetic head position thereafter becomes impossible or the access time of the magnetic disk storage device varies device by device.